University of Colorado Alpine Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Colorado Alpine Observatory clear sky chart from 2005-11-06 to 2025-05-26.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	4 6% 2 6% 4 4 4 4 6% 8% 52%	118096
	Night	2 5% 2 5% 3 3 4 3 5% 8% 60%	120089
Transparency	Day	27% 9% 13% 23% 20% 7%	117842
	Night	21% 8% 14% 26% 23% 7%	119933
Seeing	Day	13% 30% 31% 17% 7% 2	16305
	Night	9% 33% 27% 20% 10% 1	26431

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	3 3 10% 4 3 3 2 4 7% 60%	3638
	11th to 20th	Day	3 2 6% 5% 5% 4 2 5% 8% 62%	3799
	21st to end	Day	6% 3 6% 4 2 3 3 5% 7% 61%	4510
February	1st to 10th	Day	5% 4 8% 4 3 3 4 6% 10% 53%	1800
	11th to 20th	Day	3 2 5% 3 3 4 3 7% 10% 58%	1888
	21st to end	Day	4 3 8% 4 3 4 3 6% 12% 54%	1650
March	1st to 10th	Day	1 6% 4 7% 3 4 4 4 5% 5% 57%	4517
	11th to 20th	Day	1 8% 6% 9% 4 4 4 4 5% 9% 46%	5000
	21st to end	Day	2 8% 3 4 3 4 4 4 6% 10% 51%	5780
April	1st to 10th	Day	2 7% 3 6% 4 4 4 4 7% 9% 49%	2400
	11th to 20th	Day	7% 12% 4 7% 4 4 5% 4 5% 8% 41%	2371
	21st to end	Day	6% 12% 3 6% 4 5% 6% 5% 7% 8% 38%	2400
May	1st to 10th	Day	8% 13% 2 6% 4 4 5% 4 7% 9% 37%	4180
	11th to 20th	Day	8% 9% 2 6% 5% 4 6% 5% 7% 10% 39%	4528
	21st to end	Day	5% 5% 1 4 4 4 5% 4 8% 10% 50%	4897
June	1st to 10th	Day	6% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	2660
	11th to 20th	Day	7% 4 1 5% 4 5% 5% 5% 6% 8% 49%	2660
	21st to end	Day	5% 5% 2 5% 4 6% 6% 4 7% 9% 47%	2660
July	1st to 10th	Day	5% 4 1 4 5% 6% 6% 4 6% 7% 52%	4430
	11th to 20th	Day	4 3 2 6% 6% 6% 7% 6% 8% 9% 43%	4430
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 7% 8% 46%	4680
August	1st to 10th	Day	6% 6% 2 7% 6% 6% 6% 5% 8% 10% 38%	4092
	11th to 20th	Day	5% 3 1 5% 5% 5% 5% 5% 7% 8% 50%	3784
	21st to end	Day	5% 4 2 6% 4 5% 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	6% 4 2 5% 4 4 4 4 6% 6% 55%	2280
	11th to 20th	Day	6% 4 2 5% 4 4 4 3 6% 7% 55%	2280
	21st to end	Day	7% 5% 1 4 3 3 3 3 4 7% 61%	2068
October	1st to 10th	Day	3 3 1 3 2 2 3 2 4 4 71%	3170
	11th to 20th	Day	4 4 2 5% 3 3 4 3 7% 8% 56%	3150
	21st to end	Day	3 5% 2 4 3 3 3 3 4 7% 62%	3351
November	1st to 10th	Day	2 4 2 5% 3 3 3 3 5% 9% 63%	1763
	11th to 20th	Day	1 4 1 4 3 3 3 2 7% 9% 63%	1800
	21st to end	Day	1 4 3 7% 4 4 3 4 7% 7% 58%	1650
December	1st to 10th	Day	2 1 4 3 3 3 2 4 7% 71%	2620
	11th to 20th	Day	1 3 1 3 3 3 3 3 7% 10% 63%	3026
	21st to end	Day	6% 3 6% 3 4 4 4 7% 9% 54%	4004
January	1st to 10th	Night	3 2 8% 4 3 4 3 5% 7% 60%	6327
	11th to 20th	Night	1 1 5% 3 4 4 3 5% 8% 67%	6225
	21st to end	Night	1 4 3 6% 4 3 3 3 4 7% 64%	6468
February	1st to 10th	Night	4 2 5% 4 3 3 2 6% 11% 59%	2600
	11th to 20th	Night	3 2 6% 4 3 4 3 6% 8% 61%	2397
	21st to end	Night	6% 3 6% 4 3 3 3 5% 11% 57%	1980
March	1st to 10th	Night	5% 3 6% 4 4 4 3 5% 7% 59%	5356
	11th to 20th	Night	1 7% 4 7% 3 3 4 3 5% 9% 55%	5000
	21st to end	Night	2 6% 2 5% 3 3 4 4 5% 8% 57%	5179
April	1st to 10th	Night	2 6% 2 5% 3 3 4 4 5% 8% 58%	2000
	11th to 20th	Night	5% 8% 2 6% 3 4 4 3 6% 7% 52%	1977
	21st to end	Night	6% 11% 3 5% 3 3 3 3 6% 8% 49%	2000
May	1st to 10th	Night	6% 10% 2 5% 3 4 4 3 5% 7% 50%	3005
	11th to 20th	Night	8% 9% 2 5% 4 4 4 4 8% 7% 45%	2597
	21st to end	Night	4 5% 1 4 4 3 4 4 5% 7% 60%	2806
June	1st to 10th	Night	4 3 2 4 3 3 3 4 7% 10% 57%	1520
	11th to 20th	Night	4 4 1 3 3 3 3 4 6% 7% 61%	1520
	21st to end	Night	3 3 1 4 3 3 4 4 7% 8% 60%	1520
July	1st to 10th	Night	3 5% 1 4 4 3 4 4 6% 7% 59%	2540
	11th to 20th	Night	3 2 1 4 3 3 5% 4 8% 11% 56%	2540
	21st to end	Night	4 3 1 3 3 3 5% 5% 7% 8% 59%	2878
August	1st to 10th	Night	4 5% 2 5% 4 4 5% 5% 8% 10% 48%	2847
	11th to 20th	Night	4 3 1 4 4 4 4 4 7% 9% 56%	3055
	21st to end	Night	3 4 1 5% 4 4 4 5% 7% 10% 54%	3487
September	1st to 10th	Night	6% 4 1 4 4 3 3 4 5% 6% 59%	1900
	11th to 20th	Night	6% 3 1 3 3 3 3 3 5% 6% 64%	1900
	21st to end	Night	6% 4 2 3 3 3 3 3 5% 5% 64%	2109
October	1st to 10th	Night	3 3 1 2 2 2 2 2 3 5% 76%	3800
	11th to 20th	Night	2 5% 1 3 3 3 3 3 5% 8% 64%	3778
	21st to end	Night	2 4 2 5% 3 3 3 3 4 6% 67%	4173
November	1st to 10th	Night	1 4 2 3 3 3 3 3 4 9% 66%	2535
	11th to 20th	Night	1 4 1 4 3 3 3 3 6% 8% 64%	2600
	21st to end	Night	2 4 2 5% 3 4 3 3 6% 8% 60%	2745
December	1st to 10th	Night	2 1 3 1 2 2 2 3 6% 77%	4570
	11th to 20th	Night	3 2 4 3 3 3 3 5% 9% 65%	5181
	21st to end	Night	7% 2 6% 3 4 5% 4 6% 9% 53%	6974

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	22% 11% 16% 26% 20% 5%	3639
	11th to 20th	Day	23% 12% 16% 18% 23% 7%	3799
	21st to end	Day	23% 7% 11% 24% 23% 11%	4510
February	1st to 10th	Day	27% 12% 20% 21% 15% 6%	1800
	11th to 20th	Day	23% 11% 16% 26% 19% 5%	1898
	21st to end	Day	28% 8% 15% 22% 20% 6%	1650
March	1st to 10th	Day	27% 7% 8% 22% 25% 10%	4517
	11th to 20th	Day	30% 8% 13% 29% 18% 3	5000
	21st to end	Day	28% 10% 18% 31% 11% 1	5780
April	1st to 10th	Day	30% 10% 13% 25% 16% 5%	2400
	11th to 20th	Day	40% 9% 12% 20% 15% 3	2374
	21st to end	Day	40% 11% 13% 24% 12% 1	2400
May	1st to 10th	Day	40% 12% 13% 18% 15% 1	4180
	11th to 20th	Day	34% 11% 15% 24% 15% 1	4532
	21st to end	Day	26% 9% 12% 28% 22% 3	4897
June	1st to 10th	Day	31% 12% 15% 21% 17% 5%	2660
	11th to 20th	Day	30% 10% 11% 15% 23% 11%	2660
	21st to end	Day	29% 10% 11% 19% 21% 9%	2660
July	1st to 10th	Day	25% 10% 9% 15% 19% 22%	4361
	11th to 20th	Day	28% 12% 13% 20% 18% 10%	4355
	21st to end	Day	30% 13% 11% 16% 13% 16%	4532
August	1st to 10th	Day	35% 13% 16% 17% 13% 6%	4078
	11th to 20th	Day	29% 10% 15% 15% 21% 10%	3821
	21st to end	Day	30% 10% 12% 15% 23% 10%	4179
September	1st to 10th	Day	27% 8% 10% 19% 24% 12%	2280
	11th to 20th	Day	27% 8% 12% 21% 24% 9%	2280
	21st to end	Day	24% 5% 9% 23% 32% 7%	2068
October	1st to 10th	Day	18% 4 6% 19% 38% 15%	3170
	11th to 20th	Day	23% 9% 10% 29% 24% 4	3150
	21st to end	Day	21% 5% 10% 34% 26% 4	3351
November	1st to 10th	Day	20% 7% 16% 22% 29% 5%	1763
	11th to 20th	Day	18% 6% 17% 30% 23% 7%	1800
	21st to end	Day	24% 6% 14% 27% 20% 9%	1650
December	1st to 10th	Day	16% 6% 13% 36% 24% 6%	2620
	11th to 20th	Day	15% 10% 15% 38% 22% 1	3024
	21st to end	Day	23% 9% 14% 22% 25% 5%	4004
January	1st to 10th	Night	24% 9% 14% 27% 20% 7%	6327
	11th to 20th	Night	18% 12% 17% 21% 22% 10%	6225
	21st to end	Night	21% 8% 14% 23% 23% 12%	6468
February	1st to 10th	Night	21% 11% 19% 25% 19% 5%	2600
	11th to 20th	Night	21% 8% 20% 29% 19% 3	2397
	21st to end	Night	24% 8% 16% 24% 21% 8%	1980
March	1st to 10th	Night	24% 7% 11% 19% 29% 10%	5356
	11th to 20th	Night	26% 7% 10% 23% 30% 4	5000
	21st to end	Night	22% 8% 20% 36% 12% 1	5179
April	1st to 10th	Night	24% 10% 15% 28% 19% 5%	2000
	11th to 20th	Night	31% 8% 15% 26% 17% 3	1977
	21st to end	Night	31% 8% 14% 29% 17% 1	2000
May	1st to 10th	Night	29% 10% 15% 31% 14% 2	3005
	11th to 20th	Night	31% 10% 16% 27% 16%	2598
	21st to end	Night	21% 8% 15% 31% 22% 3	2806
June	1st to 10th	Night	21% 7% 20% 28% 17% 6%	1520
	11th to 20th	Night	19% 7% 13% 22% 25% 14%	1520
	21st to end	Night	19% 7% 13% 24% 25% 12%	1520
July	1st to 10th	Night	22% 8% 9% 18% 23% 20%	2500
	11th to 20th	Night	17% 10% 14% 32% 19% 8%	2492
	21st to end	Night	19% 9% 16% 26% 14% 16%	2788
August	1st to 10th	Night	28% 11% 19% 19% 19% 4	2838
	11th to 20th	Night	22% 8% 23% 18% 19% 9%	3084
	21st to end	Night	23% 9% 15% 20% 28% 5%	3487
September	1st to 10th	Night	24% 6% 12% 21% 27% 11%	1900
	11th to 20th	Night	20% 7% 11% 25% 26% 12%	1900
	21st to end	Night	22% 5% 10% 27% 29% 8%	2109
October	1st to 10th	Night	14% 4 7% 25% 37% 14%	3800
	11th to 20th	Night	18% 6% 12% 37% 22% 6%	3778
	21st to end	Night	18% 5% 9% 32% 30% 6%	4173
November	1st to 10th	Night	17% 7% 16% 26% 27% 7%	2535
	11th to 20th	Night	17% 6% 16% 31% 24% 6%	2600
	21st to end	Night	23% 8% 11% 29% 22% 7%	2745
December	1st to 10th	Night	11% 6% 14% 36% 27% 6%	4570
	11th to 20th	Night	15% 8% 14% 37% 25% 2	5182
	21st to end	Night	24% 10% 16% 21% 25% 4	6974

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 44% 24% 11% 5% 1	300
	11th to 20th	Day	14% 54% 19% 11% 1	300
	21st to end	Day	10% 52% 28% 8% 2 1	330
February	1st to 10th	Day	19% 58% 19% 2 1 1	300
	11th to 20th	Day	18% 58% 17% 5% 2	300
	21st to end	Day	22% 53% 18% 6%	249
March	1st to 10th	Day	16% 50% 25% 9% 1	304
	11th to 20th	Day	18% 37% 33% 10% 2	500
	21st to end	Day	19% 44% 29% 6% 2	550
April	1st to 10th	Day	17% 44% 30% 8% 1	500
	11th to 20th	Day	22% 43% 27% 6% 2	492
	21st to end	Day	17% 35% 36% 10% 2	500
May	1st to 10th	Day	19% 31% 32% 13% 4	607
	11th to 20th	Day	19% 20% 38% 17% 6% 1	725
	21st to end	Day	15% 23% 31% 22% 9% 1	760
June	1st to 10th	Day	6% 18% 35% 26% 12% 3	680
	11th to 20th	Day	8% 24% 36% 22% 9% 2	680
	21st to end	Day	7% 19% 36% 21% 14% 3	680
July	1st to 10th	Day	7% 8% 32% 32% 17% 4	671
	11th to 20th	Day	5% 7% 37% 30% 16% 4	668
	21st to end	Day	7% 9% 30% 33% 17% 4	666
August	1st to 10th	Day	9% 13% 35% 32% 9% 1	487
	11th to 20th	Day	7% 14% 39% 26% 13% 2	477
	21st to end	Day	8% 15% 37% 27% 11% 2	539
September	1st to 10th	Day	7% 16% 37% 28% 10% 3	490
	11th to 20th	Day	6% 17% 45% 22% 8% 1	490
	21st to end	Day	12% 19% 35% 21% 10% 3	300
October	1st to 10th	Day	8% 29% 31% 19% 9% 4	300
	11th to 20th	Day	10% 41% 31% 12% 4 2	300
	21st to end	Day	14% 44% 28% 11% 3	330
November	1st to 10th	Day	10% 49% 25% 10% 6%	300
	11th to 20th	Day	18% 45% 27% 7% 2 1	300
	21st to end	Day	14% 45% 30% 10% 1	300
December	1st to 10th	Day	17% 48% 24% 8% 2 1	300
	11th to 20th	Day	15% 56% 22% 7%	300
	21st to end	Day	20% 49% 20% 8% 2	330
January	1st to 10th	Night	12% 45% 24% 13% 4 1	1000
	11th to 20th	Night	14% 50% 22% 10% 4 1	950
	21st to end	Night	9% 50% 28% 10% 3 1	880
February	1st to 10th	Night	12% 54% 22% 9% 3	800
	11th to 20th	Night	10% 51% 26% 12% 2	784
	21st to end	Night	10% 54% 25% 10% 2	660
March	1st to 10th	Night	9% 46% 29% 13% 4	800
	11th to 20th	Night	10% 35% 31% 19% 5%	800
	21st to end	Night	11% 41% 31% 14% 3 1	751
April	1st to 10th	Night	12% 36% 34% 16% 2	600
	11th to 20th	Night	11% 37% 29% 16% 7%	593
	21st to end	Night	13% 28% 33% 18% 7% 1	600
May	1st to 10th	Night	12% 20% 38% 21% 9% 1	600
	11th to 20th	Night	17% 12% 31% 29% 11% 1	591
	21st to end	Night	9% 16% 31% 29% 14% 1	634
June	1st to 10th	Night	5% 8% 30% 35% 20% 2	570
	11th to 20th	Night	5% 14% 37% 31% 12% 1	570
	21st to end	Night	4 9% 24% 38% 22% 3	570
July	1st to 10th	Night	5% 3 18% 38% 31% 4	555
	11th to 20th	Night	1 3 22% 36% 32% 5%	552
	21st to end	Night	5% 4 12% 39% 34% 5%	624
August	1st to 10th	Night	5% 5% 19% 43% 26% 3	567
	11th to 20th	Night	7% 5% 18% 41% 25% 5%	564
	21st to end	Night	5% 6% 19% 37% 28% 5%	627
September	1st to 10th	Night	7% 4 27% 36% 22% 4	570
	11th to 20th	Night	8% 13% 26% 33% 17% 3	570
	21st to end	Night	8% 16% 30% 29% 14% 3	589
October	1st to 10th	Night	6% 27% 28% 24% 12% 2	760
	11th to 20th	Night	7% 35% 31% 19% 7% 2	760
	21st to end	Night	9% 40% 31% 13% 6% 2	865
November	1st to 10th	Night	7% 47% 29% 12% 4 1	975
	11th to 20th	Night	11% 49% 29% 8% 3	1000
	21st to end	Night	8% 44% 32% 13% 2	1000
December	1st to 10th	Night	13% 46% 29% 10% 2	1000
	11th to 20th	Night	12% 50% 25% 10% 3	1000
	21st to end	Night	12% 49% 25% 10% 4	1100

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